The "heat shock" response of Drosophila melanogaster, which is probably a homeostatic response to stress, is also a useful model system for studying fundamental questions of gene expression. During the heat shock only a few genes are transcribed. This permits us to study what appears to be a small set of coordinately controlled genes. The primary transcripts an cytoplasmic RNAs complementary to these genes are being characterized and their relationship determined. During the heat shock, translation of pre-existing mRNAs is suspended. An in vitro translation system has been developed from Drosophila cells and will be used to analyze this translational control. We have developed a technique for obtaining myogenic mass development in mass cultures of dissociated gastrula-stage Drosophila embryo cells. These cultures are being used to characterize muscle proteins and the mRNAs that code for these proteins. Attempts will be made to isolate muscle-specific RNAs for gene mapping by in situ hybridization. Cultured Drosophila cells of the Schneider line 2 contain an inducible virus. We have isolated this virus and intend to characterize it fully.